The Unified Extensible Firmware Interface (UEFI, pronounced as an initialism U-E-F-I or like "unify" without the n) is a specification that defines a software interface between an operating system and platform firmware. UEFI replaces the Basic Input/Output System (BIOS) firmware interface originally present in all IBM PC-compatible personal computers, with most UEFI firmware implementations providing legacy support for BIOS services.